1. Field of the Invention
The present invention relates to a filler metal for an aluminum brazing sheet for heat exchangers. More particularly, the present invention relates to a filler metal for an aluminum brazing sheet for heat exchangers which prevents or controls occurrence of a melting hole during heating for brazing, and to a method of manufacturing the same.
2. Description of Background Art
An aluminum alloy is generally used for automotive heat exchangers such as radiators, heaters, condensers, and evaporators due to its light weight and good thermal conductivity. In the manufacture of heat exchangers made of an aluminum alloy, a method of forming an aluminum alloy plate material or extrusion material into a specific shape, and assembling and joining the materials by brazing is generally employed.
In the case of forming a tube or a tank using a plate material, a brazing sheet in which an Al—Si alloy filler metal such as an Al—Si alloy filler metal or an Al—Si—Mg alloy filler metal is clad on one side or both sides of the plate material is used. An assembled heat exchanger is inserted into a brazing furnace and heated at an Al—Si eutectic temperature (577° C.) or more. This causes the filler metal clad on the surface of the plate material to be melted and flow toward the joint. The melted filler metal causes gaps to be filled therewith or forms a fillet at the joint, whereby joining by brazing is completed. During brazing, the filler metal is gradually melted as the temperature is increased, and immediately flows toward the joint. Dissolution of a core material into the melted filler metal progresses as the the filler metal is melted. However, progress of the dissolution is usually inhibited due to flowing of the filler metal.
In recent years, the thickness of each member of automotive heat exchangers has been decreased from the viewpoint of saving of energy and resources. To deal with a decrease in the thickness, improvement of corrosion resistance has become an urgent subject. In particular, it is important to secure corrosion resistance of a tube or a tank through which a refrigerant is passed. Therefore, it is necessary to develop a material capable of securing a corrosion perforation life equal to that of a conventional material even if the thickness of the members of heat exchangers is decreased. However, in the case of using a thin brazing sheet in which the thickness of the filler metal is small, local melting progresses in the direction of the thickness (depth) of the brazing sheet during heating for brazing. This results in occurrence of a melting hole, whereby the corrosion perforation life is decreased. In an extreme case, a through hole is formed in the brazing sheet due to the progress of melting, thereby resulting in leakage failure.
The present inventors have conducted studies on local melting occurring in the brazing sheet during heating for brazing. As a result of various experiments, the present inventors have found that a coarse Si particle present in the Al—Si alloy filler metal or Al—Si—Mg alloy filler metal causes local melting to occur. Specifically, the Al—Si alloy filler metal and Al—Si—Mg alloy filler metal have a structure consisting of an Al—Si eutectic and an Al solid solution α phase. A coarse Si particle shown in FIG. 1 (black rectangular crystallized product in the eutectic structure shown in FIG. 1) is inevitably crystallized in the eutectic structure. In particular, in the case where the size of the coarse Si particle is large, the coarse Si particle may exceed the thickness of the filler metal clad on the surface of the brazing sheet and reach the core material, as shown in FIG. 2. A melting hole shown in FIG. 3 occurs when such a material is brazed.
In the case of assembling and heating a fin material and a brazing sheet in which the thickness of the filler metal is small, if the coarse Si particle is present in the filler metal, the filler metal melts and flows toward the joint and a molten filler metal produced to surround the coarse Si particle also flows toward the joint section, as shown in FIG. 4. As a result, eutectic melting between the coarse Si particle and the core material progresses in the direction of the depth (thickness). Therefore, the larger the size of the coarse Si particle, the deeper the melting hole. In a conventional material in which the thickness of the filler metal layer is large, a large amount of molten filler metal is generated near the coarse Si particle and flows toward the joint. This prevents the molten filler metal generated to surround the coarse Si particle from immediately flowing toward the joint, whereby the coarse Si particle is rarely allowed to remain. In this case, since the coarse Si particle and the core material are dissolved into the molten filler metal, the melting hole does not progress only in the direction of the depth. However, if the size of the Si particle is remarkably large, the melting hole reaches the deep portion of the core material. This gives rise to problems relating to the corrosion resistance life.
The mechanism by which the coarse Si particle is crystallized in the cast structure of the Al—Si alloy having a hypo-eutectic composition has not been fully clarified. Moreover, refinement of the coarse Si particle and prevention of the crystallization of the coarse Si particle remain unclear. The present inventors have tried the following three measures to refine the coarse Si particle or prevent the crystallization of the coarse Si particle. As a result, the present inventors have found that these measures are effective in preventing the crystallization or growth of the coarse Si particle.
(1) A method of adding elements effective for refining the eutectic structure on the assumption that the coarse Si particle is an abnormal substance of Si (Si crystallized in layers) in the eutectic structure or a primary crystal Si particle, thereby preventing crystallization or growth of the coarse Si particle.
(2) A method of limiting impurities such as P or Fe, which becomes a nucleus of the coarse Si particle by using a high purity ground metal in the production of the Al—Si alloy filler metal, thereby preventing crystallization or growth of the coarse Si particle.
(3) A method of increasing the cooling rate when casting the filler metal, thereby controlling growth of the coarse Si particle.
The present invention has been achieved based on the above findings. An object of the present invention is to provide a filler metal for an aluminum brazing sheet for heat exchangers capable of preventing or controlling the occurrence of a melting hole during heating for brazing, and a method of manufacturing the same.